Serious mouse lag on secondary computer

EDIT: Problem (mostly) fixed; it turns out MP was actually using the WiFi on my laptop, in stead of the ethernet connection.

Since a month or two, I've been experiencing serious mouse lag on my secondary PC which has basically made me stop using MP at all.
It's only now that I decided to look into it again because I didn't need it that much until now. 

My primary PC is my work PC (at the hospital that I work), and my secondary PC is my own laptop, connected to an extra monitor at work. 
Both computers have their own IP on the network, and they connect without any trouble. The laptop is connected via an ethernet cable.

However, since a month or two, when I move my mouse to the 2nd screen, it lags immensely. Movement is still sort of smooth, with some staggering, but mostly it just lags behind, like there's a delay of about 1 second.

I've found this topic online: https://forums.stardock.com/377887 which suggests some option called "Enable if your mouse seems to move once a second" option under Advanced user settings. However, I was unable to find this option in the current version of MP (this topic is about 7 years old).

If anyone knows anything I can try to fix this, that would be much appreciated. If any of the devs could help me out, or maybe want to have a look at my computer(s) via TeamViewer or something, that's also fine!

Thank you for your time. 

EDIT: It seems MP was actually running over the WiFi connection of my laptop, even though everything else was using the ethernet connection.
I'm now trying to get it to work over the ethernet cable (I guess this must have worked in the past, before I started having lag)